The utility of intumescent sheets as a packing for mounting and positioning automotive catalytic converter monoliths is well known. The intumescent sheet is utilized as a mounting material by expansion in situ. The expanded sheet then holds the ceramic core or catalyst in place in the container or canister. The thermal stability and resilience of the sheet after expansion protects the core from thermal and mechanical shocks by compensating for the difference in thermal expansion of the metal canister and the ceramic substrate, vibration transmitted to the fragile device, and for irregularities in the metallic or ceramic surfaces. Thermal and/or mechanical shocks may cause deterioration of the ceramic substrate, which, once started, quickly accelerates and renders the device useless.
The performance of the intumescent sheet used in the aforementioned catalytic converter application can generally be characterized by TMA (Thermal Mechanical Analysis). The test consists of placing a sample of the intumescent sheet in a thermo-mechanical analyzer under load and monitoring the thickness change as a function of increasing temperature. Typically, a 50 psi (pounds per Square Inch) load is applied and thickness change is monitored from ambient to approximately 1000.degree. C. at a temperature ramp rate of 10.degree.-20.degree. C./min. Important TMA performance values include maximum negative expansion, initial expansion temperature, maximum expansion achieved, and the extent of expansion at 800.degree. C.
It is generally understood by those skillful in the art that minimizing negative expansion, decreasing initial expansion temperature, increasing maximum expansion, and maintaining a substantial percentage of the maximum expansion are all desirable TMA characteristics. For example, minimal negative expansion and a low initial expansion temperature reduces the risk of damage to the fragile monolith during initial automobile start-up, increasing maximum expansion raises the holding pressure thus better securing the monolith in the canister, and maintaining a substantial percentage of the maximum expansion sustains a sufficient holding pressure on the monolith. Holding pressure is the pressure generated by the intumescent sheet as it expands in the gap between monolith and metal housing. Insufficient sheet performance as measured by these criteria can result in mechanical and/or thermal shock to the catalyst support and subsequent loss of converter functionality.
Industrial vermiculite is commonly used as the expandable component in intumescent sheets. Vermiculite is a micaceous mineral, chemically identified as a hydrated magnesium-aluminum-iron silicate and characterized by a layered structure which exfoliates or expands in one dimension when heated at high temperatures or subjected to various chemical treatments, so as to be increased to many times its original size. Industrial vermiculite occurs naturally in an unexpanded state, and is mined in conventional manner. Since vermiculite as it is mined in the form of vermiculite ore is associated with other minerals as impurities, the crude vermiculite ore, after being reduced to particle size, has generally been beneficiated by various concentrating methods well known in the art wherein the gangue material is separated from the vermiculite particles as much as possible, and the vermiculite screened into a number of component sizes. Vermiculite has generally been regarded as the basic constituent contributing to the increased holding pressures and high temperature resiliency of intumescent sheets used in catalytic converters (Merry, R. P. and Gulati, S. T., "Design Considerations for Mounting Material for Ceramic Wall-Flow Diesel Filters", SAE Paper 811324, 1981).
U.S. Pat. No. 3,916,057 discloses an intumescent sheet comprising unexpanded vermiculite, inorganic fibrous materials, and binders suitable for such a purpose. One disadvantage of this sheet is that it undergoes negative expansion when exposed to temperatures from approximately 100.degree. C. to 400.degree. C. U.S. Pat. No. 4,305,992 partially overcomes this deficiency by disclosing an intumescent sheet comprised of ammonium exchanged vermiculite, inorganic fibrous materials, and binders. The negative expansion in this region is generally improved, initial expansion temperature is decreased by approximately 70.degree.-180.degree. C., and the maximum expansion of the material increases. However, ammonium exchange is costly, and disposal of the ammonium solutions must be monitored closely as such a solution is considered potentially harmful to the environment. U.S. Pat. No. 5,079,280 discloses a further improvement in the negative expansion region and in initial expansion temperature for intumescent sheet materials by using vermiculite subjected to potassium nitrate solution such that ion exchange with potassium occurs. Since potassium nitrate can be recycled as a fertilizer without risk to the environment, it is desirable from a disposal standpoint of view. However, a costly ion exchange process is still necessary to reduce both negative expansion and initial expansion temperature.
Furthermore, one other drawback of the exchange treatment is that the treatment diminishes the high temperature (e.g. expansion at 800.degree. C.) performance of intumescent sheets.
More stringent air emission requirements have initiated a trend to hotter automotive exhaust systems. Achieving the above desirable TMA characteristics and the higher temperature durability requirements for expanded intumescent sheets has become more difficult.
The intent of this invention is to develop intumescent sheets with improved maximum negative expansion, maximum expansion characteristics and high temperature stability without the need for an ion exchanged expandable component.